Chapter 7, Problem 7.134MP

Brass is an approximately 4:1 alloy of copper and zinc, along with small amounts of tin, lead, and iron. The mass percents of copper and zinc can be determined by a procedure that begins with dissolving the brass in hot nitric acid. The resulting solution of Cu²⁺ and Zn²⁺ ions is then treated with aqueous ammonia to lower its acidity, followed by addition of sodium thiocyanate (NaSCN) and sulfurous acid (H₂SO₃) to precipitate copper(I) thiocyanate (CuSCN). The solid CuSCN is collected, dissolved in aqueous acid, and treated with potassium iodate (KIO₃) to give iodine, which is then titrated with aqueous sodium thiosulfate (Na₂S₂O₃). The filtrate remaining after CuSCN has been removed is neutralized by addition of aqueous ammonia, and a solution of diammonium hydrogen phosphate [(NH₄)₂HPO₄] is added to yield a precipitate of zinc ammonium phosphate (ZnNH₄PO₄). Heating the precipitate to 900 °C converts it to zinc pyrophosphate (Zn₂P₂O₇), which is weighed. The equations are

1. (1) Cu(s) + NO₃⁻(aq) $\stackrel{}{\to }$ Cu²⁺(aq) + NO(g) (in acid)
2. (2) $\begin{array}{l}{\text{Cu}}^{2+}(aq)+{\text{SCN}}^{-}(aq)+HS{O}_3{}^{-}(aq)\stackrel{}{\to }\\ \text{CuSCN}(s)+HS{O}_4{}^{-}(aq)(inacid)\end{array}$
3. (3) Cu⁺(aq) + IO₃⁻(aq) $\stackrel{}{\to }$ Cu²⁺(aq) + I₂(aq) (in acid)
4. (4) I₂(aq) + S₂O₃²⁻(aq) $\stackrel{}{\to }$ I⁻(aq) + S₄O₆²⁻(aq) (in acid)
5. (5) ZnNH₄PO₄(s) $\stackrel{}{\to }$ Zn₂P₂O₇(s) + H₂O(g) + NH₃(g)
6. (a) Balance all equations.
7. (b) When a brass sample with a mass of 0.544 g was subjected to the preceding analysis, 10.82 mL of 0.1220 M sodium thiosulfate was required for the reaction with iodine. What is the mass percent copper in the brass?
8. (c) The brass sample in part (b) yielded 0.246 g of Zn₂P₂O₇. What is the mass percent zinc in the brass?